Revealing the Self-Degradation Mechanisms in Methylammonium Lead Iodide Perovskites in Dark and Vacuum

Organic-inorganic lead halide perovskite phases segregate (and their structures degrade) under illumination, exhibiting a poor stability with hysteresis and producing halide accumulation at the surface.In this work, we observed structural and interfacial dissociation in methylammonium lead iodide (CH3NH3PbI3) perovskites even under dark and vacuum conditions. Here, we investigate the origin and consequences of self-degradation in CH3NH3PbI3 perovskites stored in the dark under vacuum. Diffraction and photoelectron spectroscopic studies reveal the structural dissociation of perovskites into PbI2, which further dissociates into metallic lead (Pb-0) and I-2(-) ions, collectively degrading the perovskite stability. Using TOF-SIMS analysis, AuI2- formation was directly observed, and it was found that an interplay between CH3NH3+, I-3(-), and mobile I- ions continuously regenerates more I-2(-) ions, which diffuse to the surface even in the absence of light. Besides, halide diffusion causes a concentration gradient between Pb-0 and I-2(-) and creates other ionic traps (PbI2-, PbI-) that segregate as clusters at the perovskite/gold interface. A shift of the onset of the absorption band edge towards shorter wavelengths was also observed by absorption spectroscopy, indicating the formation of defect species upon aging in the dark under vacuum.